1. Field of the Invention
The present invention relates to a method of controlling a self-levelling device by supplying air to or discharging air from an air spring supporting a table on which a machine is mounted, for maintaining the table at a reference position.
2. Prior Art
Various precision machines or apparatus such for example as optical devices, precision measurement instruments, apparatus for fabricating ICs, and electron microscopes are susceptible to slight vibrations or displacements while in operation, and tend to malfunction or yield defective products when under vibratory conditions. To isolate such vibrations, it has been proposed to place a machine 1 (FIG. 1 of the accompanying drawings) on a self-levelling device on vibration isolation device 2 by a table 3 supported on an air spring. As disclosed in U.S. Pat. Nos. 2,956,761; 3,115,944; 3,730,473; and 4,057,212, the air spring has a valve openable and closable to admit air into or discharge air from the air spring in response to an amount of displacement of the table from a reference position, as detected by a plunger held in contact with the underside of the table. When the valve is actuated, the air spring causes the table to move upwardly or downwardly for correction of the level of the table.
The prior vibration isolation devices include a complex mechanism for isolating the displacement-detecting plunger from vibrations from the floor. Nevertheless, the table is still subjected to some types of vibrations transmitted through the plunger. When the table is displaced widely from the reference position, the plunger is also moved a large distance causing an increased amount of air to be supplied to the air spring, whereupon the table position is corrected relatively rapidly. When the amount of displacement of the table is relatively small, the valve opens slightly and the table is caused to return to its reference position in a longer period of time.